On the surface of vascular cells, ENPP1 (the protein encoded by the ectonucleotide pyrophosphatase–phosphodiesterase 1 gene) converts ATP to AMP and pyrophosphate (PPi), and CD73 converts AMP to adenosine and inorganic phosphate (Pi). Pyrophosphate inhibits calcification, tissue-nonspecific alkaline phosphatase (TNAP) degrades pyrophosphate, and adenosine inhibits TNAP. Deficiency of CD73 results in decreased adenosine levels, eliminating the inhibition of TNAP from the pathway either directly or by way of adenosine receptor signaling. Increased TNAP from the pathway activity results in decreased pyrophosphate and increased cell calcification.